1. Field of the Invention
This invention relates to a method of outputting an electron microscope image, and to an apparatus for carrying out the method. This invention particularly relates to a method of outputting an electron microscope image so that no difference in density arises at boundaries between partial images in the case where a plurality of partial images of a specimen are combined to output an overall electron microscope image of the specimen, and to an apparatus for carrying out the method.
2. Description of the Prior Art
There have heretofore been known electron microscopes for obtaining an enlarged image of a specimen by use of an electric field or a magnetic field to refract an electron beam passing through the specimen. In an electron microscope, a diffraction pattern of the specimen is formed on a back focal plane of an objective lens by the electron beam passing through the specimen, and the enlarged image of the specimen is formed by interference of the diffracted waves. The enlarged image (transmitted image) of the specimen is observed when the enlarged image is projected by a projection lens. Also, when the diffraction pattern on the back focal plane is projected, an enlarged diffraction pattern of the specimen is observed. In the case where an intermediate lens is disposed between the objective lens and the projection lens, the aforesaid enlarged image (transmitted image) or the diffraction pattern may be obtained as desired by adjusting the focal length of the intermediate lens.
In general, in order to view the enlarged image or the diffraction pattern formed in the manner as mentioned above (both the enlarged image and the diffraction pattern are hereinafter generically referred to as the electron beam image), heretofore a photographic film has been disposed at the plane of image formation of the projection lens to expose the photographic film to the electron beam image, or an image intensifier has been used for intensification and projection of the electron beam image. However, the method wherein photographic film is used has the drawback that the sensitivity of the photographic film with respect to the electron beam is low and troublesome development processing is necessary. On the other hand, the method wherein an image intensifier is used is disadvantageous in that image sharpness is low and the image is readily distorted.
Also, the electron beam image mentioned above is often subjected to image processing such as gradation processing, frequency response enhancement processing, density processing, subtraction processing or addition processing, reconstruction of a three-dimensional image by Fourier analysis, image analysis for conversion of an image into two-value system or for grain size measurement, or diffraction pattern processing for analysis of crystal information or for measurement of the grating constant, transition and grating defects. In such a case, a microscope image obtained by development of a photographic film has heretofore been detected by use of a microphotometer and converted into electric signals, which are then subjected to A/D conversion or the like and processed by computer. Such operations are very troublesome.
In view of the above circumstances, the applicant proposed in Japanese Unexamined Patent Publication Nos. 61(1986)-51738 and 61(1986)-93539 a novel method of recording and reproducing an electron microscope image wherein the electron microscope image is recorded and reproduced with a high sensitivity and with a high image quality, and wherein electrical signals carrying the electron microscope image are obtained directly so that various kinds of processing are facilitated. Basically, the proposed method of recording and reproducing an electron microscope image comprises the steps of (i) exposing a two-dimensional sensor such as a stimulable phosphor sheet for electron beam energy storage to an electron beam passing through a specimen in a vacuum to have the electron beam energy stored on the two-dimensional sensor, then (ii) exposing the two-dimensional sensor to light or heating it to release the stored energy as light emission, (iii) photoelectrically detecting the emitted light to obtain image signals, and (iv) reproducing the electron beam image of the specimen by use of the image signals.
The aforesaid two-dimensional sensor is formed of a material that is capable, upon exposure to an electron beam, of temporarily storing at least a part of electron beam energy, and which can then, upon excitation from the exterior release at least a part of the stored energy in a detectable form such as light, electricity or sound. A stimulable phosphor sheet as disclosed in, for example, U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395 is particularly suitable for use as the two-dimensional sensor. Specifically, when certain kinds of phosphors are exposed to a radiation such as an electron beam, they store a part of the energy of the radiation. When the phosphor which has been exposed to the radiation is then exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. By "stimulable phosphor sheet" is meant a sheet-shaped recording material comprising the aforesaid stimulable phosphor. In general, a stimulable phosphor sheet is composed of a supporting material and a stimulable phosphor layer overlaid on the supporting material. The stimulable phosphor layer is comprised of an appropriate binder and the stimulable phosphor dispersed therein. A stimulable phosphor layer that is self-supporting can by itself form the stimulable phosphor sheet. Examples of a stimulable phosphor for constituting the stimulable phosphor sheet are described in detail in Japanese Unexamined Patent Publication No. 61(1986)-93539.
It is also possible to use a thermal phosphor sheet as disclosed in, for example, Japanese Patent Publication Nos. 55(1980)-47719 and 55(1980)-47720 as the two-dimensional sensor. The thermal phosphor sheet is a sheet-shaped recording material comprising a phosphor (thermal phosphor) which releases the stored radiation energy as thermal fluorescence mainly by the heat effect.
With the aforesaid method of recording and reproducing an electron microscope image wherein an electron microscope image is stored on a two-dimensional sensor such as a stimulable phosphor sheet, the electron microscope image can be recorded at high sensitivity. Therefore, the intensity of the electron beam used for the recording of the electron microscope image can be decreased, minimizing damage to the specimen. With this method of recording and reproducing an electron microscope image, it also becomes very easy to carry out image processing of the electron microscope image such as gradation processing and frequency response enhancement processing. By feeding the electrical signals to a computer, it also becomes possible to carry out diffraction pattern processing, reconstruction of a three-dimensional image, and an image analysis for image conversion into a two-value system or the like simply and quickly.
On the other hand, an overall electron microscope image of a specimen is often output by combining a plurality of partial images of the specimen. Such a method of outputting an electron microscope image is widely used when, for example, the image of the specimen is to be viewed at a high resolution and over a wide range.
The output of a composite image has heretofore been carried out by a method wherein, first, hard copies of electron microscope images of respective portions (divisions) of a specimen are produced, and the composite image is formed by joining the hard copies together. However, when the composite image is thus output, differences in density arise at boundaries between adjacent partial images, making the composite image unsuitable for viewing purposes.
Such a problem is caused mainly by slight variations in the amounts of exposure to the electron beam among the recording steps for the respective partial images. When a two-dimensional sensor as mentioned above is used, the aforesaid problems are also produced by variations in the amount of exposure to light or to the amount of heat for image read-out among the respective partial images. When different two-dimensional sensors are used for the respective partial images, variations in sensitivity among the two-dimensional sensors also may constitute a cause of the aforesaid problems.